maybe this chart will help
http://www.televue.com/engine/TV3b_p...o#.VCkS5WeSyNA
Note that the equations still apply and the extra magnification factor will be different for different Barlows, because it depends on the distance that the Barlow is inside the prime focal plane, the distance from the Barlow to the eyepiece focal plane and the (negative) focal length of the Barlow.